Numerous techniques have been proposed for authenticating valuable documents (e.g., banknotes), objects (e.g., jewelry, paintings), and suchlike. For example, NL 6,613,250 suggests using a chemical test for recognizing printed documents as genuine, and U.S. Pat. Nos. 4,451,521, 6,344,261 and 6,383,618 suggest various luminescent techniques for the authentication of valuable/security documents.
A magnetic resonance technique (electron paramagnetic resonance—EPR, also known as electron spin resonance—ESR) for authenticating or identifying papers of value is described in U.S. Pat. No. 4,376,264, which suggests using substances having EPR characteristics detected by high field EPR in microwave band (starting from 1 GHz). In another method described in U.S. Pat. No. 5,149,946, radio-frequency band ESR authentication technique is proposed.
U.S. Pat. No. 5,175,499 describes techniques for authenticating a resonant material using microwave or radio frequency source to emit excitation electromagnetic radiation. Some more recent patent literature publications, such as CH 662194, WO 02/084608, and DE 10118679, propose use of EPR (electron paramagnetic resonance) and FMR (ferromagnetic resonance) for the purpose of identification and authentication of papers of value and other objects.
The aforementioned techniques are based on a variety of magnetic resonance phenomena. These phenomena are associated with nuclear, electron, atomic or molecular magnetic dipole moments acting individually or cooperatively in the presence of external magnetic fields to yield nuclear magnetic resonance (NMR), electron spin/paramagnetic resonance (ESR/EPR), ferromagnetic resonance (FMR), and suchlike.
Magnetic resonance phenomena are exhibited when magnetic dipole moments processing in a magnetic field absorb and re-radiate microwave or radio-frequency electromagnetic radiation at, or very close to, the precession frequency. A main disadvantage in utilizing the magnetic resonance phenomena for the purposes of authenticating and/or identifying objects is the necessity of an external static magnetic field for the excitation and detection of a resonance response. This external static magnetic field is usually applied either by a large electro- or superconducting magnet over the entire interrogation volume, or by a small permanent or semi-permanent magnet element placed close to the interrogated resonant material. For obtaining resonance responses with high sensitivity and high resolution, the applied external static magnetic field should be rather strong (of about, or greater than, 10 milliTesla) and be a homogeneous field. These requirements complicate both the arrangement of the marker/label, and the design of the probing device used to interrogate the resonant material carried by the marker/label. Further, the presence of a strong external magnetic field within the interrogation volume, or near the marker, may present health hazards (e.g., interference with operation of live supporting equipment such as pacemakers and suchlike), as well as the risk of wiping out data contained on magnetic media. These deficiencies render the task of identifying credit/bank cards, or other information carriers sensitive to external magnetic fields, extremely difficult.
U.S. Pat. No. 5,986,550, assigned to the same assignee as the present application and is hereby incorporated by reference, suggests marking/labeling objects for identification and authentication of the objects using techniques based on mixed electron-nuclear resonances. In this technique the precession of nuclear electric or magnetic dipole moments is provoked by cooperative or individual electronic subsystems.